This invention relates to flat-panel displays of the cathode-ray-tube (xe2x80x9cCRTxe2x80x9d) type.
A flat-panel CRT display basically consists of an electron-emitting device and a light-emitting device. The electron-emitting device, commonly referred to as a cathode, contains electron-emissive regions that emit electrons over a relatively wide area. The emitted electrons are appropriately directed towards light-emissive elements distributed over a corresponding area in the light-emitting device. Upon being struck by the electrons, the light-emissive elements emit light that produces an image on the display""s viewing surface.
The electron-emitting and light-emitting devices are connected together to form a sealed enclosure maintained at a pressure much less than 1 atm. The exterior-to-interior pressure differential across the display is typically close to 1 atm. In a flat-panel CRT display of significant viewing area, e.g., at least 10 cm2, the electron-emitting and light-emitting devices are normally incapable of resisting the exterior-to-interior pressure differential on their own. Accordingly, a spacer (or support) system is conventionally provided inside the sealed enclosure to prevent air pressure and other external forces from collapsing the display.
The spacer system typically consists of a group of laterally separated spacers positioned so as to not be directly visible on the viewing surface. The presence of the spacer system can adversely affect the flow of electrons through the display. For example, electrons can occasionally strike the spacer system, causing it to become electrically charged. The electric potential field in the vicinity of the spacer system changes. The electron trajectories are thereby affected, commonly leading to degradation in the image produced on the viewing surface.
Numerous techniques have been investigated for making a spacer system electrically invisible to the electron flow. For example, see U.S. Pat. Nos. 5,532,548 and 5,675,212. Although many of these techniques significantly reduce image degradation caused by a spacer system, some image degradation can still occur as the result of electron deflections caused by the spacer system. Making a spacer system completely electrically invisible to the electron flow is extremely difficult. Accordingly, it is desirable to have a technique for reducing image degradation despite undesired electron-trajectory changes caused by a spacer system.
In accordance with the invention, the intensity at which electrons emitted by a first plate structure in a flat-panel display strike an oppositely situated second plate structure in the display for causing the second plate structure to emit light is controlled in a manner to reduce image degradation that could otherwise arise from undesired electron-trajectory changes caused by effects such as the presence of a spacer system between the plate structures. The first plate structure contains an electron-emissive region for emitting electrons. The second plate structure contains a light-emissive element for emitting light upon being struck by electrons.
Electrons emitted from the electron-emissive region strike the light-emissive element with an intensity having an electron-striking centroid along the second plate structure. The resultant light is emitted by the light-emissive element with an intensity having a light-emitting centroid along the second plate structure. The light-emitting centroid is shifted in a primary direction due to shifting of the electron-striking centroid in the primary direction. The shifting of the electron-striking centroid in the primary direction occurs because electrons are generally deflected in the primary direction, typically due to the presence of the spacer system. Deflection of electrons in the primary direction and the resultant shift of the electron-striking centroid in the primary direction can also arise from various errors in fabricating the display.
A useful parameter for characterizing centroid shifting in the primary direction is primary centroid shift ratio Rp defined as (a) the amount of shift of the light-emitting centroid in the primary direction divided by (b) the amount of shift of the electron-striking centroid in the primary direction. In one aspect of the invention, primary centroid shift ratio RP is no more than 0.5 when the magnitude of shift of the electron-striking centroid in the primary direction is in a suitable range. By having shift ratio RP be this low, the shift of the light-emitting centroid in the primary direction is only a fraction, typically a small fraction, of the shift of the electron-striking centroid in the primary direction. Any such shift of the electron-striking centroid arising from electron deflections caused, for example, by the spacer system is therefore significantly inhibited from causing a shift in the light-emitting centroid and producing image degradation.
When centroid shifting can occur in a further direction different from, typically perpendicular to, the primary direction, another useful parameter is relative centroid shift ratio RP/RF for centroid shifting in the primary direction relative to centroid shifting in the further direction. Item RP is the primary centroid shift ratio dealt with above. Item RF, the further centroid shift ratio, is (a) the amount that the light-emitting centroid is shiftable in the further direction divided by (b) the amount that the electron-striking centroid is shiftable in the further direction. In another aspect of the invention, relative centroid shift ratio RP/RF is no more than 0.75 when the magnitudes of shift of the electron-striking centroid in the primary and further directions are in suitable ranges.
Arranging for relative centroid shift ratio RP/RF to satisfy the foregoing criteria takes advantage of the fact that the average magnitude of electron deflections is normally considerably greater in the primary direction than in the further direction. In particular, the presence of the spacer system typically does not cause the electron-striking centroid to shift significantly in the further direction. Consequently, electron deflections which occur do not lead to significant image degradation. With primary centroid shift ratio RP being no more than 0.5 under the indicated conditions and with further centroid shift ratio RF being relatively high under the indicated conditions so that relative centroid shift ratio RP/RF is no more than 0.75 under the indicated conditions, the flat-panel display operates quite efficiently in the further direction in producing light as the result of electrons striking the second plate structure.
In a further aspect of the invention, the intensity of electrons striking the light-emissive element along an imaginary plane extending in the primary direction through the center of the light-emissive element generally perpendicular to the second plate structure has a 10% moving average intensity profile having a local minimum. A 10% moving intensity average in a particular direction across the light-emissive element means that the intensity employed to characterize a particular point of the light-emissive element is the average intensity along a line centered on that point and of a length equal to 10% of the mean dimension of the light-emissive element in the particular direction. Use of a 10% moving average smoothes out large local intensity variations, including those resulting from measurement errors, in the actual electron-striking intensity so as to produce a highly characteristic representation of the electron-striking intensity.
The intensity value at the local minimum in the 10% moving average profile for the electron-striking intensity is normally no more than 95%, typically no more than 90%, of the maximum intensity value in the 10% moving average profile. By having such a local minimum in the 10% moving average intensity profile, primary centroid shift RP is no more than 0.5 when the magnitude of shift of the electron-striking centroid in the primary direction is in a suitable range. Similarly, relative centroid shift ratio RP/RF is normally no more than 0.75 when the magnitudes of shift of the electron-striking centroid in the primary and further directions are in suitable ranges. Any such shift of the electron-striking centroid arising from electron deflections caused, for example, by the spacer system is therefore significantly inhibited from causing a shift in the light-emitting centroid and producing image degradation.
The present flat-panel display typically contains a two-dimensional array of electron-emissive regions and a like-arranged two-dimensional array of light-emissive elements. As a result, intensity averaging across multiple light-emissive elements can be substituted for a moving intensity average across one light-emissive element. Using this alternative averaging approach, the intensities of electrons striking the light-emissive elements along imaginary planes extending in a primary direction through the centers of the light-emissive elements have a composite average intensity profile which has a local minimum. Similar to the local minimum in the 10% moving average electron-striking intensity profile, the local minimum in the composite average electron-striking intensity profile for multiple light-emissive elements leads to significant reduction in the amount of average shift of the light-emitting centroids, thereby substantially reducing image degradation.
In yet another aspect of the invention, an electron-emissive region of a flat-panel display contains a plurality of laterally separated electron-emissive portions which selectively emit electrons. The display includes a system for focusing electrons emitted by the electron-emissive portions. The electron focusing system has a corresponding plurality of focus openings located respectively above the electron-emissive portions. The electrons emitted by the electron-emissive portions respectively pass through the focus openings.
A light-emissive element, which is situated opposite the electron-emissive region and therefore opposite all of its electron-emissive portions, emits light to produce at least part of a dot of the display""s image upon being struck by electrons emitted from the electron-emissive portions. By utilizing electrons that pass through plural focus openings to produce at least part of an image dot in this manner, the display can readily achieve the above-mentioned intensity characteristics. The display""s image is much improved. The invention thereby provides a substantial advance.